1. Field of the Invention
The present invention relates to a circularly polarized wave antenna having a patch antenna structure suitable for being mounted on a vehicle or the like, and more particularly, to a circularly polarized wave antenna made of sheet metal, which leads to reduction in cost.
2. Description of the Related Art
A vehicle-mounted antenna has an advantage of transmission or reception being performed using circularly polarized wave signals, making it unnecessary to control polarized directions of waves even during movement of a vehicle. Therefore, a small circularly polarized wave antenna having a patch antenna structure has been widely used as such a vehicle-mounted antenna.
FIG. 6 is a plan view showing a conventional example representative of a circularly polarized wave antenna of this type (for example, see Japanese Unexamined Patent Application Publication No 2000-151261 (Page 2, FIG. 5)). In the antenna device 1 shown in FIG. 6, which is referred to as the circularly polarized wave antenna, a radiating conductor (a patch electrode) 3 is provided on one surface of a dielectric substrate 2 using metal film forming technique such as printing or the like, and a ground conductor (not shown) is provided on most of the other surface of the dielectric substrate 2. The radiating conductor 3 has a substantially square shape and is comprised of degenerative separation elements 4 and 5 which are formed by cutting out a pair of corners of the conductor opposite in phase to each other. One end of a feed pin 6 is passed through the dielectric substrate 2 and the ground conductor, and is soldered to a predetermined feeding point in the radiating conductor 3, and the other end of the feed pin 3 is connected to a feeding circuit (not shown).
In the antenna device 1 generally constructed as such, predetermined high-frequency signals are supplied to the radiating conductor 3 via the feeding pin 6, such that the radiating conductor 3 resonates to radiate radio waves. In the radiating conductor 3, the resonant length of the resonance mode, in the diagonal direction in which the degenerative separating elements 4 and 5 exist, is shorter than the resonant length of the resonance mode in another diagonal direction orthogonal to the diagonal line. Accordingly, the size (the cutout area) of the degenerative separation elements 4 and 5 is appropriately adjusted to set a phase difference of about 90 degrees between both resonant modes, such that a synthesized dominant mode of both the resonant modes is excited. This enables the antenna device 1 to operate as a circularly polarized wave antenna.
The aforementioned conventional circularly polarized wave antenna (antenna device 1) is constructed such that the radiating conductor 3 is provided on one surface of the dielectric substrate 2 and the feed pin 6 is connected to the radiating conductor 3. This construction has a problem in that the antenna device 1 of this type cannot be manufactured at a low cost because the dielectric substrate 2 with a small amount of dielectric loss is expensive. In addition, the process of forming the radiating conductor 3 using the metal film forming technique is also complicated. Particularly, when the resonant frequency is high, a dielectric material with an extremely small dielectric loss is required as a material of the dielectric substrate 2 in order to secure the efficiency of the antenna. In this case, the dielectric material is very expensive and results in high material cost. For example, when a circularly polarized wave antenna having a resonant frequency of 5.8 GHz for an electronic toll collection (ETC) system is manufactured utilizing the aforementioned technique, the antenna device may become extremely expensive.